322 



LOWER INVERTEBRATES. 



Okdee IV. — SCUTIBRANCHIA. 



All the remaining Gasteropoda contrast with the Zygobranchiata in the fact that 

 the torsion of the body has caused the obsolescence or abortion of one of the true gills, 

 and for this reason Dr. Lankester lias arranged them under one ordinal head Azygo- 

 branchia. When, however, we take other characters into consideration, it becomes 

 necessary to divide up this large group, and in the following jiages the Scutibranchia, 

 Ctenobranchia, and Heteropoda together equal tlie Azygobranchia of that able Eng- 

 lish morphologist. 



The iirst family, the Teochid-s;, are commonly known as top-shells, the shell of the 

 typical forms, when inverted, being strikingly similar to the plaything of our youth. 

 — _„^,^ The shell is spiral, and is either pyra- 

 midal or turbinated, and has a nacreous 

 interior. Many of these shells are sold 

 as ornaments, after the epidermis and 

 external layers of the shell have been 

 cut away, leaving the whole a mass of 

 mother-of-pearl. The animal has long 

 and slender tentacles, and at the bases 

 of these arise the peduncles which sup- 

 ])ort the eyes. .The head, and si^es of 

 the body, are ornamented with fringed 

 lobes and longer tentacles. When the 

 animal withdraws into its shell, it closes 

 the ajterture with an o]:)erculum, which 

 may be either horny, or calcareous with a horny base. As the animal increases in size, 

 the ojierculum also grows liy additions which are arranged in a spiral. When crawling 

 about, the animal carries its operculum on the dorsal surface of the foot, as do all oper- 

 culated gasteropods. Some of the ojiercula of the smaller species are in great repute 

 as eye-stones. Their whole value in this respect is due to the fact that they have no 

 irregularities which would injure the cornea or the inner surface of tlie eyelid. Other- 

 wise they are no better than any other hard substance of similar shape and size. The 

 physiology of their action is readily understood. The 

 species live in shallow water near the shore and are 

 herbivorous in their diet. 



In Trochiis, the shell forms a regular pyramid, and 

 the base is flattened. The whorls are flattened, the ajier- 

 turo is olilique, and tlie operculum is horny and multi- 

 spiral. Of this genus and its various sub-divisions, over 

 two hundred and fifty species have been described. In 



^^ our northern waters, these forms are represented by several species of 



^1^ the genus 3Iurf)arita, which in many respects is intermediate between 



Mar- Trochus and Turbo. The whorls of the shell are moi-e ventricose, or 



swollen, than in Trockiis, and the thin epidermis allows the pearly shell 



to be readily seen. The species are found from extreme low-water mark to a depth of 



one hundred fathoms and over. 



In Turbo the whorls are ventricose, the aperture large and rounded, and the 



Fig. 399. — TVoc/ms zhyphinus. 



Fig. -loo. — 

 (/arita 



